Centralized traffic controlling system for railroads



June 3%, 1936. N. D. PRESTON 2;@45723 CENTRALIZED TRAFFIC CONTROLLING SYSTEM FOR RAILROADS Filed Aug. 18, 1935 B in B .5 22m UN 35:00

Patented June 30, 1936 UNITED STATES CENTRALI'ZED TRAFFIC CONTROLLING SYSTEM FOR RAILROADS Neil D. Preston, Rochester, N. Y., assignor to General Railway Signal Company, Rochester,

Application August 18, 1933, Serial No. 685,765

11 Claims.

This invention relates to centralized trailic controlling systems for railroads and it more particularly pertains to the means employed for interrelating the operation of a plurality of field stations with the operation of the communication part of such a system.

In that type of centralized traffic controlling system where a communication system of the station selective type is employed, control impulses are transmitted from the control office to the several field stations during separate operating cycles. Similarly, indication impulses are transmitted to the control office in accordance with the particular station which obtains access to the communication line and in accordance with the positions of the various trafilc controlling devices at the associated station.

In such a centralized trailic controlling system, where only one field station may be in communication with the control ofiice at any one time and where the communication system is normally at rest, it is necessary to provide means for storing the fact that a change in the condition or position of the traflic controlling devices at a field station has occurred, so that such a change may initiate the system for the transmission of indications from that particular field station in accordance with the new conditions.

In a centralized traflic controlling system of the type outlined above, it may occur that the stored condition of a change at a field station is maintained for an obnormal period of time, due either to failure of the system to properly transmit the indications or to the continued operation of one or more devices at the station. Either case results in that station monopolizing the indication transmitting line so that other stations with changed indications to transmit are prevented from obtaining use of the communication line, when such other stations are of inferior order as determined by the look-out arrangement provided in the system.

In view of the above and other considerations, the present invention proposes to provide means whereby a changed condition at a field station may be suitably stored, with this storing means having associated therewith other means which prevents that field station obtaining access to the communication system until a succeeding cycle is allotted to the use of another field station, after which the station which is attempting to monopolize the communication line may again obtain access to the communication system. In a modified form of the present invention means 55 are pro-vided which prevents a field station obtaining access to the communication system after it has once transmitted its indications until a condition exists when no other station in the system has new indications to transmit.

Other objects, purposes and characteristic features of the present invention will be in part obvi- 0115 from the accompanying drawing and in part pointed out as the description of the invention progresses.

In describing the invention in detail reference will be made to the accompanying drawing which illustrates in a diagrammatic manner the apparatus and circuits employed at the control oflice and at a single field station in accordance with the present invention and associated with a communication system indicated as comprising two line circuits connecting the field station with the control oifice.

A centralized traflic controlling system organized as contemplated in accordance with the present invention is considered as comprising a control oflice located at a convenient point and a plurality of field stations located throughout the territory under the control of an operator in the control (mice. The communication system connecting the control office with the single field station illustrated, and extending to other field stations in the system. may be of any suitable type but for convenience in disclosing the present invention it is assumed that such communication system is of the station selective duplex coded type as disclosed, for example, in the pending application of T. J. Judge et al., Ser. No. 640,062, filed October 28, 1932, corresponding to Australian Patent 14,323 of 1933. It is to be understood however, that the specific form of communication system is not essential to the functioning of the present embodiment, but rather the means disclosed in the present invention may be considered as an adjunct to any type of communication system.

In the communication system contemplated as used in connection with the present invention, a series stepping and control line circuit connects the control office with the several field stations and receives control impulses from a suitable source of energy located in the control ofiice. A series message or indication line circuit also receives energy from a suitable source of energy located in the control oflice and this line circuit line in the upper left hand portion of Fig. 1. A

portion of the apparatus at a typical field station is illustrated in the right hand portion of Fig. 1 and is shown connected to the control oflice by the control line conductor [0, the control line return conductor i2, indication line conductor I4, and indication return line conductor [6.

A modified form of the invention is illustrated in Fig. 2 of the drawing.

This system is of. the station selective type, the station being selected by receiving control ime pulses which energizes the station relay SO associated with that station. Similarly, whenever a lock-out relay such as relay L0 is energized at a particular station, this station may control the impulsing of the indication line conductors l4 and H5 in accordance with the indications to be transmitted on each step from this particular station. The energization of relay LO causes the system to be immediately initiated into a cycle of operations. 7

During the application of each control impulse to the stepping and control line conductor [0, the contacts of a line repeating relay F are positioned in accordance with the polarity of the control impulses for the selection of the required station and for the transmission of controls to this station after it is selected.

Since a description of the operation of the stepping relay bank as well as the station selecting operation of the system is unnecessary for an understanding of the present invention, only those relays and contacts have been illustrated which are considered necessary to show the manner in which the present invention is applied to the system fully disclosed in the above mentioned application, Ser. No. 640,062.

A change relay CH is provided which responds to a change in condition of the various traflic controlling devices located at the associated station. A slow acting relay SA is picked up at the beginning of each cycle of operations and is maintained energized until the end of such cycle. A slow acting detained lock relay DL is provided, which relay is picked up at the start of a cycle only at the station which obtains access to the communication system. This relay hassuch slow acting characteristics that it maintains its armature in its attracted position between successive cycles, when the cycles follow each other in rapid succession as determined by the fact that a station is waiting to obtain access to the communication line when a particular cycle of operations is terminated. In other words, relay DL will be dropped at the end of a. cycle, during which it has been energized, if there is no other station waiting with indications to transmit. If there is another station waiting then the succeeding cycle is started in such short order that relay BL is not dropped between cycles.

The communication part of the system includes at the field station a three-position biased-toneutral polar line relay F together with its quick acting line repeating relay F 3 Quick acting neutral relay SE repeats the operations of slow acting relay SA that is, relay SB is picked up at the beginning of a cycle and is dropped at the end of the cycle. At each field station suitable compensating resistances H3 and 2R are provided to compensate for the difference in distance between the field station at which they are located and the control office. In other words, the field station at the end of the system requires no resistances but for each station nearer the control office, higher resistances are employed so that the same value of. current from the indication line battery IB always flows through the indication line circuit irrespective of the station which is transmitting.

Resistance 2R is employed during the transmission of indications and resistance IR is employed during the lock-out period at the beginning of a cycle of operations. Relays PB and PF are used to control the impulsing of the indication line circuit as determined by contact N3 of relay FP in its back and front positions respectively.

That part of the control ofiice illustrated shows more particularly the portion of the apparatus ginning of each cycle and is dropped at the end Associated with relay SA is a neusimiof the cycle. tral slow acting repeating relay ZSA havin lar characteristics.

Associated with the line relay and the line repeating relays is a bank of stepping relays (not shown) provided to mark off the steps of each cycle of operations. An impulsing relay EP is controlled by the stepping relays for governing the time spacing of theimpulsing applied to control line circuit. is provided which repeats the condition of relay EP for the purpose of selecting the front and back branches of the indication line circuit in the control ofiice.

The polarity of the impulses applied to the control line circuit from the control battery CE is determined by code sending relays PC NC. These relays are controlled in accordance with the positions of the code jumpers and control levers which are rendered efiective for a particular cycle of operations by the corresponding code determining relay associated with the station to be selected. 1

Two neutral message receiving relays MB and -MF are energized over the message or indication line circuit at each step. The conditions of these relays are stored in suitable pilot and indication storing relays but for the purpose of this dis- A branch dividing relay DV closure it is considered necessary to show only the energizing circuits of these relays, since these cirabove mentioned Australian Patent 14,323 of 1933. Most of the reference characters applied to the present drawing correspond to those used the above mentioned application. The exceptions those which relate to the circuit of the detained lock relay DL, which relay provides the fundamental feature of the present invention and its cooperation with the complete communication system.

' Operation The system of the present invention is normally in a passive condition, or condition of rest, from which it may be initiated into its active condition either from the control ofiice or from any of the field stations whenever there are new controls or new indications to be transmitted. If several field stations have indication-s ready for transmission at the same time these indications are transmitted from such field stations to the control ofiice, one station at a time during separate operating cycles.

Normal comditioms.-With the system normally at rest, the change relay CH is maintained energized through the usual track circuit relay contacts and the contacts of the usual switch repeating relay, which relays and circuits have not been included in the present drawing since the organization of such circuits is well known in the art.

In order that the communication system may be initiated into operation from each of the several field stations of the system, the message or indication circuit, including line conductors I4 and I6, is normally connected to battery 13 in the, control ofilce.

Automatic transmission. of indications.-Any change in the indication conditions at a field station as, for example, the occupancy of the detector track section or a change in position of the track switch, is indicated by the track relay or the switch repeating relay respectively causing the deenergization of relay CH Relay CH drops its contact I54 which closes a circuit for picking up relay L0 extending from (-l-), back contact I01 of relay FP back contact I38 of relay S13 back contact I48 of relay DL, back contact I53 of relay SA upper winding of relay L0 and back contact I54 of relay CH to Relay L0 applies potential through its front contact I55 to the right hand terminal of its upper Winding for maintaining the above described circuit complete even though relay CH is again picked up. The purpose of this substitute potential is to maintain the look-out relay in its picked up position after relay SA has been picked up and throughout the remainder of the cycle, during which cycle relay CH is resensitized by being again energized.

The closure of front contact I56 of relay L0 completes the indication line circuit through this particular field station over a circuit extending from the terminal of battery IB in the control oflice, back contact I51 of relay DV, lower winding of relay MB, back contact I58 of relay ZSA, indication line wire I4, front contact I56 of relay L0 back contact I12 of relay SB lower winding of relay L0 compensating resistance lR indication line return wire I6 and thence to the terminal of battery IB.

It will be apparent that, should the change have occurred at some other field station, the indication line circuit would have been closed at that station through a front contact similar to contact I55 of the associated lock-out relay and a back contact similar to I12, through the lower winding of the associated lockout relay and the compensating resistance to indication line conductor I6. At any rate, the energization of the indication line circuit, including the lower winding of the message receiving relay MB, picks up relay MB and initiates the system into a cycle of operations in a manner which is fully disclosedin the above mentioned Australian Patent 14,323 of 1933.

The initiation of the system, irrespective of whether controls are to be transmitted or not,

results in the synchronous operation of the stepping relays at the control ofiice and at each field station. The application of the first stepping impulse to the control line conductor II] causes relays F'P SA and SE to be energized. Relays SA and SE remain energized throughout the cycle while relay FP repeats each stepping impulse.

Lock-out between field statioms.Since this invention is applied to a system in which a plurality of field stations are connected by means of a common communication circuit to the control o-ifice, it becomes necessary to provide means whereby only one station at any one time can actively associate itself with the communication circuit for the transmission of indications. The particular lock-out arrangement is the same as disclosed in the above mentioned Australian Patent 14,323 of 1933 but in order to show the cooperation of the present invention with the lockout organization of a centralized trafiic controlling system, the lock-out feature will be briefly explained.

It may happen that a change in the indication conditions may occur at several field stations at the same time. In order to prevent more than one station actually interrupting the indication line circuit during a particular cycle of opera tions, the look-out relay of only one station can 1 be in its picked up position after a cycle has been initiated. The lock-out means provided at each field station is effective to select that station nearest the control oflice, having new indications to transmit at the beginning of a cycle of operations.

This system has what may be termed an initiating period and a lock-out period at the beginning of each cycle. The initiating period may be said to include the time which elapses between the occurrence of a change in conditions at a field station, up to the instant that the quick acting line repeating relays (such as relay FP at the field stations pick up their contacts in response to the first impulse applied to the control line circuit.

The. lock-out period'may be considered as that time which elapses between the actual picking up of the contacts of the FP relays at the associated stations and the picking up of their respective slow acting SA relays, similar to relay SA In the event that two change relays such as relay CH drop their contacts at substantially the same time, it will be apparent that two lockout relays such as relay L0 will be energized over circuits including the upper windings of the look-out relays.

Assuming that relay L0 is picked up at the same time that a similar lock-out relay at a station farther out the communication line is picked up, the indication line wire I4 is connected to the indication line return wire 16 at the indicated field station by reason of the closed contact M35 of relay L0 The other lock-out relay, at the more distant station, cannot close an initiating circuit because of the open back contact I56 of relay L0 During the lock-out period relay L0 will be selected, since it will be maintained energize-d subsequent to the picking up of relay FP by reason of its lower winding being included in the indication line circuit until its stick circuit is closed at the end of the lock-out period by the picking up of contact l5? of relay SA The lockout relay at the station farther out the line will drop away during the lock-out period because it has no selecting circuit completed over the indication line circuit.

Two or more stations may have changes in indication conditions which occur in succession instead of simultaneously. In such cases where the last change occurs at a station nearer the control oflice prior to the picking up of the FP relays, then the lock-out relays at the stations farther out the line will be dropped out during the look-out period, or their pick-up circuits are opened by their respective FP relays and their selecting circuits are opened by reason of the picked up lock-out relay which is closest to the control office. It is of course obvious that when the first change occurs closer to the control office, the look-out relay at such station will be selected,

On the other hand, no lock-out relay, irrespective of its location, can be picked up subsequent to the picking up of the FP relays at the beginning of a cycle, because their pick-up circuits are initially opened at back contacts similar to I01 of the various FP relays and these circuits are maintained open throughout the cycle b3 back contacts similar to I38 of the SB relays.

During an operating cycle for the transmission of indications, the step-by-step operation at the field station is dependent upon front contact I'II of lock-out relay L0 It will be understood that during a cycle for the transmission of controls when relay L0 is not picked up, the step-by-step operation at the field station is dependent upon front contact I34 of relay S0 Automatic suppression of a. station.--It will now be assumed that the station illustrated in Fig. l actively associates itself with the communication circuit for transmitting indications to the control oflice as determined by the actuation of relays L0 SA and SB. The lock-out operation, the synchronous stepping operation, the registration of the station in the control office and the transmission and recording of the indications are all effected as shown and described in the above mentioned Australian Patent 14,323 of 1933.

The operation whereby this particular station actively associates itself with the communication circuit has been briefly described above. When relay SA is picked up at the start of the cycle (with relay L0 picked up) a circuit is closed for picking up relay DL extending from front contact 2I0 of relay L0 front contact 2 of relay 5A and winding of relay DL. to With back contact I48 of relay DL picked up, the pickup circuit of relay LO (upper winding) is maintained open but relay LO remains stuck up until the end of the cycle over a circuit extending from front contact I68 of relay L0 front contact I53 of relay SA upper winding and. front contact I55 of relay L0 to At the end of this particular cycle, the system returns to its normal at-rest position as illustrated in Fig. l, with relay DL dropping after a predetermined interval of time has been measured ofi following the dropping of front contact 2II of relay SA If there is no other station waiting with indications to transmit, the dropping of contact I48 of relay DL makes it possible for the particular station illustrated to again actively associate itself with the communication circuit. If it were not for the interruption of the pick-up circuit including the upper winding of relay L0 by contact I48 of relay DL, it is obvious that this field station could continuously actively associate itself with the communication line, thereby excluding other stations located farther out the line. This condition might be occasioned because of the faulty operation of some of the circuits at this particular station, which would prevent any inferior station sending its indications.

Assuming that there is another station waiting with indications to transmit when the system reaches the end of the cycle, during which the station illustrated in Fig. 1 used the line, then at the time relay SA is dropped other similar SA relays will drop their contacts so that other stations, irrespective of their location, may have an opportunity to pick up their lock-out relays before relay DL of the illustrated station drops its contact I48. Therefore, at the end of the cycle relay DL introduces a time interval dur ing which this particular illustrated station is prevented from actively associating itself with the communication circuit. During this interval another station may actively associate itself with the communication circuit in accordance with its order of preference, as determined by its geographic location should there be more than one station waiting when the end of the cycle is reached.

From the above explanation it will be obvious that a field station, at which a condition exists which causes the station to attempt to actively associate itself with the communication circuit at the end of each cycle, can again obtain access to the communication circuit only after one intervening cycle has occurred if another station is waiting. This is because, during the second cycle when another station is permitted to use the line, relay DL will be dropped so that this particular station can actively associate itself with the line circuit to the exclusion of other inferior stations. Briefly, when such a condition exists that a particular station persists in successively actively associating itself with the communication circuit, the detained lock relay DL prevents this taking place except possibly on every other cycle as determined by whether or' not another station wants the use of the line.

Description of modification-A modification of the detained lock feature has been illustrated in Fig. 2. This circuit is exactly the same as the circuit of Fig. 1 except for the addition of front contact 2I2 on relay DL. This added front contact provides a stick circuit for relay DL as long as relay SA is picked up. This means that when a particular station, with a condition existing which causes it to attempt to use the communication circuit continuously after it has once used it, is prevented from again actively associating itself with this circuit until there are no other stations in the system with indications totransmit.

Relay DL is picked up at the start of a cycle when the particular station obtains access to the line circuit as previously described. It is stuck up over a circuit extending from front contact 2 I 2 of relay DL, front contact ZI I of relay SA and winding of relay DL, to At the end of the cycle, relay SA drops its contact 2II which allows relay DL to drop its contact if there is no other station waiting with indications ready to transmit. Therefore, this particular station can again initiate a cycle and actively associate it cations to transmit, relay SA will drop its front contact 2| II at the end of the cycle but will again pick up this front contact before relay DL has time to drop its contacts. This is because some other station initiates a cycle of operations and causes all SA relays to be picked up. During this succeeding cycle relay DL is again energized so that it measures off a similar time interval at the end of the next cycle and if another station actively associates itself with the communication circuit, relay SA is again picked up to energize relay DL before it has had time to drop its contacts. This operation continues and keeps relay DL picked up until the end of a cycle is reached when there is no station awaiting use of the line. Then relay DL will drop and the communication system can be used by this particular station.

From the above description of the operation of the system when arranged as in Fig. 2 it will be obvious that all stations will transmit their indications in rotation in accordance with their geo graphic location when several stations are ready to transmit at the same time, even if a preferred station continues to have indications ready for transmission before all other stations complete their transmission. In other words, assuming that stations I, 2 and 3 so numbered with respect to their location from the control office, have indications ready to transmit at the same time and further assuming that station l continues to have indications ready for transmission after its use of the system, the order of sequence of operation will be the transmission from stations I, Z and 3 in the order named, followed by the transmission from station l because it continues to have indications ready for transmission.

It is to be understood in connection with the present invention, that any desired means may be provided for introducing the time interval above mentioned. For example, one or more additional slow acting relays may be successively picked up after relay BL is picked up, with back contact I48 controlled by the last slow acting relay in the series. In this way any desired interval of time may be measured off to suit actual operating conditions. Similarly, a thermal time element relay could be used in place of the slow acting relay DL to introduce the time delay.

It is also to be understood that the embodiment of the present invention can be used in connection with other forms of lock-out systems instead of the geographic type illustrated in Fig. 1. For example, the superiority of code type lock-out, as effected by the operation of the SI relay disclosed in the prior application of W. D. Hailes et a1, Ser. No. 526,674, filed March 31, 1931, can be used.

Having described a centralized traffic controlling system as one specific embodiment of the present invention, it is desired to be understood that this form is selected to facilitate in the disclosure of the invention rather than to limit the number of forms which it may assume and it is to be further understood that various modifications, adaptations and alterations may be applied to the specific form shown to meetthe requirements of practice without in any manner departing from the spirit or scope of the present invention except as limited by the appended claims.

What I claim is:

1. In a remote control system, a control oflice connected to a plurality of field stations by means of a communication circuit, means at each station for transmitting messages to said office when the associated station is actively connected to said communication circuit, a lock-out relay at each of said stations for effecting the active connection of the corresponding station with said com munication circuit whereby said station trans mits messages to said oilice, a detained lock relay at each of said stations, means jointly controlled by the associated lock-out relay in its station connecting condition and said communication circuit in its message transmitting condition for controlling said detained lock relay, and means controlled by each of said detained lock relays for disabling the associated lock-out relays as long as there is a station actively connected to said communication circuit.

2. In a remote control system, a control ofiice connected to a plurality of field stations by means of a communication circuit, means at each station for transmitting messages to said ofiice when the associated station is actively connected to said communication circuit, a lock-out relay at each of said stations for effecting the active connection of the corresponding station with said communication circuit whereby said station transmits messages to said oifice, a detained lock relay at each of said stations, pick-up circuits for said detained lock relays, means jointly controlled by the associated lock-out relays in their station connecting conditions and said communication circuit in its message transmitting condition for energizing said pick up circuits, stick circuits for said detained lock relays, means controlled by said communication circuit for energizing said stick circuits, and means controlled by each of said detained lock relays for disabling the associated lock-out relay as long as. there is. a station actively connected to said communication circuit.

3. In a remote control system, a control ofiice and. av plurality of field stations, a lookout relay at each station, means for simultaneously energizing a plurality of said lockout relays, means for maintaining one and only one of said lockout relays energized during an operating cycle of said system, means for transmitting messages from one of said stations to said oifice when the associated lockout relay is operated, a slow acting relay, means for operating, said slow acting relay,

and means controlled by said slow acting relay for rendering the operating circuit. of said associated lockout relay ineffective for a comparatively long time intervalv at. the end of a cycle during which said associated? lockout. relay was maintained operated.

4. In a remote control system, a control office, a first field station, a plurality of other field stations, a communication system connecting said office with said stations, means including said communication system for transmitting indications from each of said stations to said office in response to a change in condition at the corresponding station, a slow acting relay energized in response to the active association of said first station with said communication system, means controlled by said slow acting relay for measuring 01f a predetermined time interval after said first station is disassociated from active use of said communication system, and means including said slow acting relay for preventing said first station obtaining active association with said communication system as long as any one of said other stations is transmitting indications or as long as any one of said other stations initiates the transmission of indications before said time interval has been measured oil.

5. In a remote control system, a control oiiice and a plurality of field stations connected by a line circuit, a lockout relay at each station, a first local circuit for the lockout relay at each station, a second circuit including said line circuit for the lookout relay at each station, means for energizing one and only one of said lockout relays by way of said first and said second circuits during an operating cycle of said system, means for energizing another of said lockout relays by way of its first circuit only during said cycle, means for transmitting messages from one of said stations to said oiiice only during the cycle that the associated lockout relay is energized by way of its first and its second circuits, and means for rendering the first circuit or" the associated lockout relay ineffective for a comparatively long time interval at the end of a cycle during which said associated lockout relay was energized by means of its second circuit.

6. In a remote control system, a control o ifice and a plurality of field stations, a lockout relay at each station, means for operating one of said lockout relays during an operating cycle of said system, means for transmitting messages from one of said stations to said office when the associated lockout relay is operated, a slow acting relay, means including said slow acting relay for rendering the operating circuit of said associated lockout relay ineffective for a comparatively long time interval at the end of a cycleduring which said associated lockout relay was operated, and means for maintaining said slow acting relay operated when once operated until the end of the cycle during which it is operated.

'7. In a communication system, a control office and a plurality of stations connected by a line circuit, means for operating said system through cycles of operations, means including a lookout relay at each of said stations for initiating said system into cycles of operations by actively associating the corresponding station with said communication system, means including the lookout relay at a particular station for preventing all other stations from being actively associated with said communication system, a time measuring device at each of said stations, means responsive.

to the active association of a station with said communication system for operating said device, and means including said device for preventing the associated station from actively associating itself with said communication system as long as any one of said stations is actively associated with said communication system.

8. In a communication system, a control office and a plurality of stations connected by a line circuit, means for operating said system through cycles of operations, means including a lockout relay at each of said stations for initiating said system into cycles of operations by actively associating the corresponding station with said communication system, ,means including the lockout relay at a particular station for preventing all other stations from being actively associated with said communication system, a time measuring device at each of said stations, means responsive to the active association of a station with said communication system for operating said device, and means including said device for preventing the associated station from ac tively associating itself with said communication system until a predetermined time after said communication system completes a cycle of operations.

9. In a communication system, a control office and a plurality of stations connected by a line circuit, means for operating said system through cycles of operations, means including a lockout relay at each of said stations for initiating said system into cycles of operations by actively associating the corresponding station with said communication system, means including the lockout relay at a particular station for preventing all other stations from being actively associated with said communication system, a time measuring device at each of said stations, means responsive to the active association of a station with said communication system for operating the associated device, means including said device for preventing the associated station from actively associating itself with said communication system as long as any one of said stations is actively associated with said communication system, means including the devices at said plurality of stations for allotting all other stations a preferred chance to actively associate themselves with said communication system irrespective of the condition existing which requires the association of said particular station with said system.

10. In a communication system, a control office, a superior station and an inferior station connected to said control office by a line circuit,

means for operating said system through cycles 0 of operations, means including a lookout relay at each of said stations for initiating said system into cycles of operations and actively associating the corresponding station with said communication system, means including the lookout relay at said superior station for preventing said inferior station from being actively associated with said communication system, a time measuring device at said superior station, means responsive to the active association of said superior station with said communication system for operating said device, and means including said device for preventing said superior station from actively associating itself with said communication system as long as said inferior station is actively associated with said communication system.

11. In a communication system, a control office, a superior station and an inferior station connected to said control ofiice by a line circuit, means for operating said system through cycles of operations, means including a lockout relay at each of said stations for initiating said system into cycles of operations and actively associating the corresponding station with said communication system, means including the lookout relay at said superior station for preventing said inferior station from being actively associated with said communication system, a time measuring device at said superior station, means responsive to the active association of said superior station with said communication system for operating said device, means including said device for preventing said superior station from actively associating itself with said communication system as long as said inferior station is actively associated with said communication system, means including the devices at said plurality of stations for allotting said inferior station a chance to actively associate itself with said communication system irrespective of a condition existing which requires the active association of said superior station with said communication system.

NEIL D. PRESTON.

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